1. Field of the Invention
The present invention relates to a disk drive apparatus, and more particularly to a structure for fixing a hub to the outer periphery of a spindle motor.
2. Description of Related Art
In a conventional disk drive apparatus, the spindle motor is used as the drive mechanism for rotating a disk. The head of the disk drive apparatus is maintained at a predetermined distance above the disk and positioned at a desired track to read the data recorded on the track or write data to the track. If the rotation of a disk is abnormal (e.g., deviates from the normal rotation around a fixed axis), the tracing of the track by the head may be difficult to perform and an error may occur in the reading or writing of data. This type of error may be referred to as runout. Accordingly, one of the basic performance requirements is to provide consistent high-speed rotation around a fixed axis that minimizes or reduces the likelihood of runout of the disk surface.
Recently, hard disk drives with several disks are used to increase the recording capacity of disk drives. As the number of disks increases, the hub on which the disks are mounted requires a wider surface on which the disks are to be fixed. By increasing the number of disks, the load on the motor also increases. In order to withstand this increased load, an outer ring rotation type motor may be used to support both ends of the shaft. Typically, an outer ring rotation type motor includes a fixed shaft, which is the central axis, and a hub mounted on the outer periphery of the shaft. The hub rotates around the fixed shaft through ball bearings. The disks fixed on the outer periphery of the hub rotate along with the hub.
FIG. 1 shows a cross-sectional view of a conventional outer ring rotation type motor. According to FIG. 1, the outer peripheral surface of the hub 81 is wide enough to mount a plurality of disks. The hub 81 is mounted to the shaft 82, which provides a fixed axis. The ball bearings allow the hub 81 to rotate about the shaft 82. The rotational axis of the hub 81 typically coincides with the central axis of the shaft 82. However, various factors, which are-difficult to control, may affect the normal rotation of the disk. For example, the positioning of the ball bearings, the bonding between a ball bearing and the hub, and the adjustment of the balance of the upper and lower ball bearings.
Errors resulting from the abnormal rotation of the disk may be reduced or eliminated by integrating the shaft, ball bearings, and sleeves into a bearing cartridge. FIG. 3 shows a bearing cartridge. A bearing cartridge 20 typically includes a shaft 13, a sleeve 11, a plurality of ball bearings 12, and a retainer 10 for holding the ball bearings 12 on a ring at equal intervals. Grooves in the shaft 13 and the sleeve 11 are used to hold the ball bearings at a fixed location.
FIG. 2 illustrates a spindle motor with a bearing cartridge 20. The spindle motor includes the bearing cartridge 20, a hub 14 on which disks are to be mounted, a stator 17, a magnet 15, a flange 16, and a yoke 19. The magnet 15 and the yoke 19 are fixed to the hub 14. The inner periphery of the hub 14 is fixed to the outer periphery of the sleeve 11 of the bearing cartridge. The stator 17 is fixed to a flange 16 and is disposed between the shaft and the magnet 15. By fitting the central hole of the disk over the hub 14, the disk is fixed to the hub 14. The hub 14 rotates about the shaft 13 integrally with the sleeve 11.
Generally, the hub is formed of aluminum, and the sleeve of the bearing cartridge is formed of stainless steel. As the temperature changes during the operation of the disk drive, stress due to the difference of thermal expansion between the hub and the sleeve is generated. This stress often causes distortion in the hub.
Generally the disk drive apparatus operates within a temperature range of about 5xc2x0 C. to 55xc2x0 C. If distortion occurs in the hub within this temperature range, a thermal runout (i.e., runout due to a temperature change) occurs. The difference in the coefficient of linear expansion between the hub and the sleeve may cause the hub to change in shape. For example, the dotted lines shown in FIG. 4 illustrates the deformation of the hub 14. When the disk mounted on the hub is rotated, such deformation of the hub generates not only an axial runout but also a runout in the direction of the disk plane, causing an error in data reading and writing.
It is desirable to reduce the deformation of a hub caused by a temperature change.
It is also desirable to reduce the stress produced by a temperature change that occurs in the contact region between the inner periphery of the hub and the outer periphery of the sleeve.
Furthermore, it is desirable to reduce the runout of a disk that occurs when the hub is deformed.
The motor in the disk drive apparatus according to this invention uses a bearing cartridge that integrates a shaft, ball bearings, and a sleeve. The sleeve, which is fixed to the outer periphery of the bearing cartridge, and the hub, integrally rotate about the shaft, which is the central axis. The inner periphery of the hub and the outer periphery of the sleeve are attached in a bonding region between the ball bearing holding portions of the bearing cartridge. At least one stress reducing region or hub is provided in the bonding portion.
In the motor in the disk drive apparatus, the diameter of the inner periphery of the hub, which is bonded to the outer periphery of the sleeve of the bearing cartridge, does not have a uniform length in the bonding region. Specifically, the inner diameter in part of the bonding region is made larger than the remaining part, and this partial region is defined as a stress reducing region. The formation of the stress reducing region enables the reduction of the stress which occurs between the outer periphery of the sleeve and the inner periphery of the hub when thermal expansion is generated by temperature change. Accordingly, the distortion of the hub is reduced.